1. Field of the Invention
The present invention relates to a piezo-actuator's displacement magnifying mechanism used, for example, in a shock absorber having its damping force variable (hereinafter referred to as the variable-damping-force shock absorber).
2. Description of the Prior Art
An example of a conventional piezo-actuator's displacement magnifying mechanism used in the variable-damping-force shock absorber is disclosed in Japanese Patent Laid-Open No. 85210/1986 (called, in Japanese language, "Tokkaisho 61-85210"), a construction of which mechanism is shown in FIG. 1.
In operation of the conventional piezo-actuator's displacement magnifying mechanism, as shown in FIG. 1, when a predetermined voltage is applied to a piezo-actuator 23 of a laminated type, the piezo-actuator 23 expands with the voltage. Such expansion of the piezo-actuator 23 slightly displaces a plunger 25 to apply a pressure to a hydraulic fluid confined in a fluid chamber 27. Such pressure is then applied to a piston 19 through the hydraulic fluid to drive the piston 19 downward, so that a damping-force control valve is controlled so as to be opened and closed. In the conventional piezo-actuator's displacement magnifying mechanism having the above construction, the plunger 25 is larger in an area (which is subjected to the pressure of the hydraulic fluid) than the piston 19. Consequently, when the plunger 25 is displaced in a longitudinal direction of the piezo-actuator 23 by a certain distance or displacement, such distance or displacement is magnified by the piston 19.
However, in the conventional piezo-actuator's displacement magnifying mechanism having the above construction, since an O-ring is mounted on each of the plunger 25 and the piston 19 to prevent the hydraulic fluid from passing through an annular clearance between: the plunger 25 and its corresponding cylinder; and that between the piston 19 and its corresponding cylinder, there is a fear that the piston 19 is displaced against a resilient force exerted by a return spring 21 under the influence of thermal expansion of the hydraulic fluid confined in the fluid chamber 27 even when no voltage is applied to the piezo-actuator 23.
In order to solve the above problem, when the O-ring mounted on the piston 19 is removed to permit the hydraulic fluid (which has been thermally expanded) to escape from the fluid chamber 27 through the annular clearance between the piston 19 and its corresponding cylinder, there is a fear that the piston 19 is not displaced even when a predetermined voltage is applied to the piezo-actuator 23 to cause the plunger 25 to apply a pressure to the hydraulic fluid (which is confined in the fluid chamber 27) so as to intend to move or displace the piston 19 downward. In this condition, even if the piston 19 happens to slightly displace, the piston 19 gradually returns to its initial position in a short time of, for example, two seconds or so.
In a conventional method for solving the above further problem, a pulse-shaped predetermined voltage is applied again to the piezo-actuator 23 to fill the fluid chamber 27 with the hydraulic fluid. However, such conventional method is poor in easiness in control, and suffers from large power consumption.
Therefore, an ideal means for solving the problems inherent in the conventional piezo-actuator and the conventional method is summarized as follows:
"Means for laying a restraint on hydraulic fluid movement only when the piezo-actuator is energized".
One of the ideal means is disclosed in Japanese Patent Laid-Open Publication No. 26041/1989 (called, in Japanese language, "Tokkaisho 64-26041"), which one lays a restraint on the hydraulic fluid movement by the use of a resilient sealing member such as O-rings and rubber sheet members only when the piston is displaced. However, such resilient sealing member is poor in durability when it is used in an opening/closing area.
The above-mentioned flow or leakage of the hydraulic fluid passing through the annular clearances between the plunger/piston and their corresponding cylinders forms a so-called annular-clearance flow of the hydraulic fluid. It is well known that a flow rate of the hydraulic fluid passing through the annular-clearance is directly proportional to a cross-sectional area of the annular clearance, while inversely proportional to both of a length of the annular clearance and the viscosity of the hydraulic fluid. There are three factors for controlling a flow rate of the hydraulic fluid passing through the annular clearance. These three factors are: the cross-sectional area of the annular clearance; length of the annular clearance; and the viscosity of the hydraulic fluid passing through the annular clearance.
The means disclosed in the above Japanese Patent Laid-Open Publication No. 26041/1989 controls a flow rate of the hydraulic fluid (which passes through the annular clearance) by controlling one of the above three factors, i.e., by controlling the cross-sectional area of the annular clearance.